The broad, long-term objective of this proposal is the molecular description of heterokaryon incompatibility in Neurospora crassa. Heterokaryon incompatibility is a genetic system that acts as a barrier to the asexual fusion of strains of unlike genotype. In a compatible encounter, two hyphae fuse and exchange cytoplasms and nuclei, which maintain their haploid state. In an incompatible encounter, the hyphae fuse but are rapidly sealed off from the rest of the hyphae and undergo a rapid degeneration. Incompatibility is determined by at least 10 unlinked loci (het genes). An allelic difference at any one of these is sufficient to evoke the incompatibility reaction. Incompatibility systems are of interest for two reasons: they are widespread among the fungi, including pathogenic forms, and they represent a fundamental molecular process by which unlike cytoplasms recognize each other and program their own destruction. The mechanism involved in the incompatibility reaction, and its regulation are being sought specifically by: 1) cloning, defining, and sequencing some or all the het genes and their heterologous alleles; 2) isolating and characterizing mutants that interfere with the incompatibility reaction, to identify genetic components of the system not apparant from the natural variant (het) alleles; 3) cloning and characterizing the genes identified by such mutants; and 4) characterizing the transcription and possibly the regulation of the het and mutant genes as they are cloned.